Aliphatic polycarbonates are known to be biodegradable and widely used for packages, coatings and others. An aliphatic polycarbonate can be prepared by copolymerizing an epoxide with carbon dioxide, which is environment-friendly since a toxic compound such as phosgene is not used. For such a process, there have been developed various types of catalysts, e.g., metallic zinc compounds.
There have recently been reported highly active binary catalyst systems comprising (Salen)Co or (Salen)Cr derivatives (wherein H2 Salen is N,N′-bis(3,5-dialkylsalicylidene)-1,2-cyclohexanediamine) combined with an onium salt such as [R4N]Cl and PPNCl (bis(triphenylphosphine)iminium chloride) or a base such as an amine and phosphine. [(Salen)Co system: (a) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang, Y.-J.; Peng, X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664; (b) Cohen, C. T. Thomas, C. M. Peretti, K. L. Lobkovsky, E. B. Coates, G. W. Dalton Trans. 2006, 23; (c) Paddock, R. L. Nguyen, S. T. Macromolecules 2005, 38, 6251; (Salen)Cr system: (a) Darensbourg, D. J.; Phelps, A. L.; Gall, N. L.; Jia, L. Acc. Chem. Res. 2004, 37, 836; (b) Darensbourg, D. J.; Mackiewicz, R. M. J. Am. Chem. Soc. 2005, 127, 14026]
In case of using a binary catalyst system comprising a (Salen)Co compound, the oxygen atom of an epoxide coordinates to the central Co atom having Lewis acid character and carbonate anion generated by the action of an onium salt or bulky amine base reacts with the activated epoxide through nucleophilic attack as shown below. With this system, the polymerization was conducted typically under the conditions that [epoxide]/[catalyst] is 2,000 and temperature is below 45° C., with the maximum turnover number (TON) of 980 and the turnover frequency (TOF) of 1400 h−1.

Coates, G. W. et al. have also developed a highly active catalyst composed of a zinc complex having a β-diketiminate ligand, which shows a high turnover rate of 1,116 turnover/hr [Coates, G. W. Moore, D. R. Angew. Chem., Int. Ed. 2004, 6618; U.S. Pat. No. 6,133,402]. Coates et al. achieved a still higher turnover rate of 2,300 turnover/hr when a zinc catalyst having a similar structure is used [J. Am. Chem. Soc. 125, 11911-11924 (2003)]. The catalytic action of the zinc complex comprising β-diketiminate ligand has been proposed to occur as shown below [Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2003, 125, 11911].

Under above-mentioned mechanisms, the catalystic systems have some drawbacks that will prevent them from being commercially available. It is conceptually difficult to achieve a high turnover number (TON) under these mechanisms. In order to achieve a high TON, the catalyst should therefore be active even at a high [monomer]/[catalyst] ratio condition. However, at this condition, the chance for the chain-growing carbonate unit to meet the coordinated epoxide is diminished, consequently resulting in a low activity.
Because all the addition polymerization reaction is exothermic, heat removal during polymerization is a key issue in designing the process. If the catalyst works at a reasonably high temperature, we can remove the heat by using ambient water or air, but if the catalyst works only at a low temperature, for instance, room temperature, we have to use some cryogen, which makes the process expensive. In a solution or bulk polymerization, the attainable conversion of monomer to the polymer is limited by the viscosity caused by formation of polymer. If we can run the polymerization at a higher temperature, we can convert more monomers to polymers because the viscosity is reduced as the temperature is increased. For the propagation mechanism shown above, the ΔS‡ is negative and the activation energy (ΔG‡) for the step increases as the temperature increases, leading to a lower activity at a higher temperature.
The TON and TOF values achieved by a binary catalyst system comprising a (Salen)Co compound or a zinc complex having a β-diketiminate ligand are still low enough to warrant to further improvement, because low activity means higher catalyst cost and higher levels of catalyst-derived metal residue in the resin. This metal residue either colours the resin or causes toxicity. While TON of 980 attained with a binary catalyst system comprising a (Salen)Co compound for CO2/(propylene oxide) copolymerization, the residual cobalt level in the resin reached 600 ppm unless it was removed.
Therefore, there has been a need to develop a catalyst which is capable of polymerizing an acyclic epoxide or a cyclic epoxide at a high rate under a high-temperature industrial condition or at a highly diluted condition, to produce a polymer having a high molecular weight.
Further, there have been made many unsuccessful attempts to recover catalysts from polymer products after polymerization, and, therefore, it is another object of the present invention to provide an effective way to recover active catalysts after use.